Regensburg 2022 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 27: Quantum Coherence and Quantum Information Systems (joint session TT/DY)

TT 27.2: Talk

Thursday, September 8, 2022, 09:45–10:00, H22

Gralmonium: Granular aluminum nano-junction Fluxonium qubit — •Dennis Rieger, Simon Günzler, Martin Spiecker, Patrick Paluch, Patrick Winkel, Lothar Hahn, Judith K. Hohmann, Andreas Bacher, Wolfgang Wernsdorfer, and Ioan M. Pop — Karlsruhe Institute of Technology, Germany

Mesoscopic Josephson junctions (JJs), consisting of overlapping superconducting electrodes separated by a nanometer thin oxide layer, provide a precious source of nonlinearity for superconducting quantum circuits and are at the heart of state-of-the-art qubits, such as the transmon and fluxonium. Here, we show that in a fluxonium qubit the role of the JJ can also be played by a lithographically defined, self-structured granular aluminum (grAl) nano-junction: a superconductor-insulator-superconductor (SIS) JJ obtained in a single layer, zero-angle evaporation. The measured spectrum of the resulting qubit, which we nickname gralmonium, is indistinguishable from the one of a standard fluxonium qubit. Remarkably, the lack of a mesoscopic parallel plate capacitor gives rise to an intrinsically large grAl nano-junction charging energy in the range of 10−100 GHz, comparable to its Josephson energy E_J. We measure average energy relaxation times of T₁=10 µ s and Hahn echo coherence times of T₂^echo= 9µ s. The exponential sensitivity of the gralmonium to the E_J of the grAl nano-junction provides a highly susceptible detector. Indeed, we observe spontaneous jumps of the value of E_J on timescales from milliseconds to days, which offer a powerful diagnostics tool for microscopic defects in superconducting materials.